"The ability of species to adapt by changing one base pair at a time on any gene, and to do so with comparative rapidity if selective advantages are reasonably large, explains the fine details of the matching of many species to their environment. It was from the careful observation of such matchings by naturalists in the mid-nineteenth century that the Darwinian theory arose. Because the observations were made with extreme care, it was highly probable that immediate inferences drawn from them would prove to be correct, as the work of Chapters 3 to 6 shows to be the case. What was in no way guaranteed by the evidence, however, was that evolutionary inferences correctly made in the small for species and their varieties could be extrapolated to broader taxonomic categories, to kingdoms, divisions, classes, and orders. Yet this is what the Darwinian theory did, and it was by going far outside its guaranteed range of validity that the theory ran into controversies and difficulties which have never been cleared up over more than a century."